1. Field of the Invention
The present invention relates a surface treatment of a glass fabric used for a fiber reinforced composite such as multilayer circuit boards.
2. Disclosure of the Prior Art
In the past, glass yarns coated with an organic compound are woven to obtain a glass fabric used to make a fiber reinforced composite such multilayer circuit boards. The organic compound is called as a sizing agent and acts as a protecting film for the glass yarns. After a weaving step the glass fabric is heated at about 400.degree. C. over a period of several tens of hours to remove the organic compound from the glass fabric. This heat treatment is known as a heat-cleaning treatment. Subsequently the glass fabric, free of the organic compound, is treated with a coupling agent to improve adhesion between the glass fabric and a resin which is used at a later stage of manufacturing the multilayer circuits boards. An organosilane compound can be used as the coupling agent. For example, the glass fabric may be dipped in a solution including the coupling agent diluted with an organic solvent or water. The resin is impregnated into the treated glass fabric to a form a prepreg. The multilayer circuit board can be manufactured by stacking and compressing a plurality of copper foils and prepregs.
When the organic compound is not sufficiently removed from the glass fabric by the heat-cleaning treatment, there arises a problem of decreasing the adhesion between the glass fabric and the resin because the glass fabric can not be uniformly,treated with the coupling agent. In addition, when carbon is left on the glass fabric as a residue of the organic compound, there is a probability of adversely influencing the insulating property of the fiber reinforced composite.
Thus, the heat-cleaning treatment is an important step necessary for producing good multilayer circuit boards. However, as described above, since the heat-cleaning treatment requires heating the glass fabric at about 400.degree. C. over the period of several tens of hours, there is a problem of providing a low production rate of the glass fabric, free of the organic compound, in spite of using a relatively large amount of energy. To improve the production rate, Japanese Patent Early Publications [KOKAI] Nos. 62-111493 and 64-1733 to 1735 propose to perform a plasma treatment to the glass fabric under a reduced pressure to remove the organic compound from the glass fabric, or perform the plasma treatment simultaneously with the heat-cleaning treatment. However, since the plasma treatment is performed under a reduced pressure, there is a problem of requiring an expensive and complex furnace with a vacuum apparatus.
On the other hand, for small-sizing electronic devices and providing high performance to the devices, it is desired to produce printed circuit boards or multilayer printed boards having high-density circuits. In particular, as a distance between through-holes or a distance between a conductive pattern of an internal layer of the multilayer printed board and a through-hole becomes to be shorter to form the high-density circuits, there arises an undesired phenomenon of deteriorating the insulating property therebetween. As one of the reasons for such a phenomenon, the influence of copper migration should be considered. The copper migration is explained as follows. That is, when an electric potential is applied between copper circuits under the condition of high humidity, copper of the circuit corresponding to an anode is dissolved, and the dissolved copper diffuses through an insulating portion and deposits on the copper circuit corresponding to a cathode, so that the insulation between the circuits is broken. When the copper migration happens at the interface between a resin and the glass fabric which is located between the through-holes or between the through-hole and the conductive pattern of the internal layer, it is called a CAF (Conductive Anodic Filaments). It is believed that the CAF is caused by an insufficient adhesion between the resin and the glass fabric. In other words, it is believed that the CAF happens through a clearance between the resin and glass fabric which is caused when the adhesion therebetween is poor, or by damages of drilling of the through-holes. Therefore, it is possible to prevent the occurrence of the CAF by improving the adhesion between the resin and glass fabric.
Consequently, a more effective surface-treatment of the glass fabric is desired to improve the above described problems.